The present disclosure generally relates to a control strategy for a vehicle torque converter for an automatic transmission. More particularly, the torque converter is controlled in such a manner as to reduce or eliminate slip in the individual speed or shift clutches of the automatic transmission.
Conventional automatic transmissions rely on a torque converter for the primary purpose of transforming a high revolutions-per-minute (rpm) and low torque operating state of an internal combustion engine to a lower rpm and higher torque state. A prime example includes when the vehicle is at a stop, in a drive gear, and at or near idle. In this case, the torque converter transforms, via a hydraulic or viscous coupling, the idle speed of the engine (typically 300-900 rpm) to a speed of 0-100 rpm. At the same time, the speed reduction is accompanied by an increase in torque. This is beneficial since a large amount of starting torque is required to accelerate the vehicle from a stationary position. Conversely, when the vehicle is traveling at a higher rate of speed, the torque converter is no longer needed and to improve efficiency a lockup clutch is provided to directly couple the engine to the transmission.
In addition, the conventional automatic transmission uses a plurality of shift clutches to switch among various gear trains, each having a specific gear ratio or speed range. Under high load conditions a great deal of torque must be transmitted through the shift clutches. If the torque exceeds the design value for the clutches or if the clutches have become worn, shift clutch slip can occur that will lead to rapid wear-out of the shift clutch and or other related components.
To reduce or eliminate this possibility, other systems have been designed to monitor clutch slip and to increase the pressure to the slipping clutch. However, these systems are often complicated, require additional components and/or separate delivery systems. Other systems use a torque converter reaction torque to develop the necessary pressure to reduce shift clutch slip. However, these systems do not provide the necessary reaction torque when the torque converter lockup clutch is in the engaged state. Moreover, these systems do not restrict torque converter lockup in the event of a slipping shift clutch.
For these reasons, an improved torque converter lockup control system and strategy is needed to aid in reducing or eliminating shift clutch slip in an automatic transmission.